According to progress of broadband in recent years, requirement for a large-capacity and high-speed radio communication has been increased. In preparation for enlargement of capacity, a third generation becomes widely used in an amplifier circuit for a base station of mobile phones, and further, it is predicted to proceed to a fourth generation. Besides, a new communication method (LTE) is also practically used, and it is predicted that the enlargement of capacity further advances in future. Accordingly, an amplifier circuit with higher powered, and higher efficiency has been required. On the other hand, for an amplifier circuit for a radar transmission/reception module, high-power and broadbanding are required to enable high-performance such as expansion of sensing distance, resolution enhancement, and so on, and further high-efficiency is required to enable reduction in operation cost and downsizing of a cooler.
An amplifier including an input terminal, an output terminal, a first transmission line connected to the input terminal, and a second transmission line connected to the output terminal is known (for example, refer to Patent Document 1). In a common-source field-effect transistor or a common-emitter transistor, a gate terminal or a base terminal is connected to the first transmission line, and a drain terminal or a collector terminal is connected to the second transmission line. A gate-bias variable field-effect transistor or a base-bias variable transistor is provided at least one of a first terminal at an opposite side of the input terminal of the first transmission line or a second terminal at an opposite side of the output terminal of the second transmission line, and is grounded via a capacitor element.
[Patent Document 1] Japanese Laid-open Patent Publication No. 05-251962
In the amplifier circuit, there is a case when wasteful electric power is consumed if a signal in a wide frequency band is amplified and efficiency deteriorates. However, to improve the efficiency, gain is lowered. It is difficult to enable both high efficiency and high gain.